This project investigates the mechanisms of initial resistance to infection and generation of protective immunity to bacteria. The immune response to Francisella tularensis strain LVS, a gram negative facultative intracellular bacterium, is being characterized in terms of the cell types involved and their products. Results to date have demonstrated mice which survive sublethal SC or ID infection are solidly immune to subsequent lethal IP or IV infection. Experiments using either mice depleted of CD4+, CD8+, or + T cells, or scid mice reconstituted with various T cell subpopulations, demonstrate that loss of either CD4+ or CD8+ T cells does not affect generation and expression of specific protective immunity to LVS; however, loss of + T cells has a minor effect on the strength. Moreover, B cell knockout mice are seriously impaired in their ability to survive secondary lethal challenge. This defect does not seem to be due to absence of antibody production, however. Cytokine production by T cells, and perhaps B cells, appears to be a major protective mechanism involved in generation of secondary immunity, but antibodies are clearly not necessary for protection. However, a minor role for protection by anti-bacterial antibodies, that is itself dependent on T cells and cytokines for expression, has been observed. A new major mechanism of protection has also been characterized in murine responses to Francisella: very strong specific protective immunity to LVS develops in both normal and nu/nu mice, but not scid mice, within 2-3 days after ID priming with LVS. These results implicate a cell type present in nu/nu mice but not in scid mice that is able to generate immunity rapidly. In fact, both in vivo depletion studies, transfer studies, and studies using knockout mice suggest a major role for B220+ cells that is independent of antibody production. This response also appears to operate in Listeria monocytogenes infection, and to be nonspecific. The bacterial antigens responsible for induction of early protection are under intense study. Intramuscular inoculation of mice with chromosomal LVS DNA, as well with oligonucleotide DNA containing unmethlyated CpG motifs, results in the generation of strong protective immunity to both early and late lethal LVS challenges. Protection against Listeria monocytogenes challenge using oligonucleotide DNA has also been demonstrated. Studies using various immunodeficient and knockout mice revealed that DNA-stimulated protection is highly dependent on lymphocytes, especially B cells. The mechanism(s) of this rapid generation of immunity may have important implications for development of vaccines to be used during community outbreaks of bacterial diseases. In addition, novel vaccine strategies are being studied in this model.